The present invention relates in general to lithography and more particularly to systems for imaging lithographic printing plates using digitally controlled laser output. More specifically, this invention relates to a novel lithographic printing plate especially suitable for directly imaging and utilizing with a wet lithographic printing press.
Traditional techniques for introducing a printed image onto a recording material include letterpress printing, gravure printing, and offset lithography. All of these printing methods require a plate. To transfer ink in the pattern of the image, the plate is usually loaded onto a plate cylinder of a rotary press for efficiency. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing cylinders, in contrast, contain a series of wells or indentations that accept ink for deposit onto the recording medium. Excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
The term xe2x80x9clithographic,xe2x80x9d as used herein, is meant to include various terms used synonymously, such as offset, offset lithographic, planographic, and others. By the term xe2x80x9cwet lithographic,xe2x80x9d as used herein, is meant the type of lithographic printing plate where the printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced. In a dry lithographic printing system that does not utilize water, the plate is simply inked and the image transferred directly onto a recording material or transferred onto a blanket and then to the recording material.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typically subject to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the image to the plate and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining and anodizing affect both the performance and the durability of the printing plate. Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are also well known in the art, and need not be further described herein. The aluminum support is thus characterized by having a porous, wear-resistant hydrophilic surface, which specifically adapts it for use in lithographic printing, particularly where long press runs are required.
The plates for an offset press are usually produced photographically. The aluminum substrate described above is typically coated with a wide variety of radiation-sensitive materials suitable for forming images for use in the lithographic printing process. Any radiation-sensitive layer is suitable which, after exposure and any necessary developing and/or fixing, provides an image, which can be used for printing. Lithographic printing plates of this type are usually developed with an aqueous alkaline developing solution, which often additionally comprises a substantial quantity of an organic solvent.
To prepare a wet plate using a typical negative-working substractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), thereby exposing the hydrophilic surface of the aluminum plate.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As is evident from the above description, photographic platemaking processes tend to be time consuming and require facilities and equipment adequate to support the necessary chemistry. Efforts have been made for many years to manufacture a printing plate, which does not require development or which only uses water for development. In addition, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic radiation, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced closely to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing xe2x80x9cdotsxe2x80x9d which collectively form a desired image (see, e.g., U.S. Pat. No. 4,911,075). Because of the ready availability of laser equipment and its amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. These systems include:
1) Argon-ion, frequency-doubled Nd-YAG and infrared lasers used to expose photosensitive blanks for traditional chemical processing, as for example described in U.S. Pat. Nos. 3,506,779; 4,020,762; 4,868,092; 5,153,236; 5,372,915; and 5,629,354. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation, with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate, as for example described in U.S. Pat. No. 4,132,168.
However, the need for high writing speeds, coupled with the constraint of the low-powered lasers favored by industry, has resulted in a requirement for printing plates that have a very high photosensitivity. Unfortunately, high photosensitivity almost always reduces the shelf life of these plates.
2) Another approach to laser imaging uses thermal-transfer materials, as for example described in U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946; and 5,395,729. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. The transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent polymer sheet with the unirradiated transfer material still on it leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic, and the acceptor material is hydrophilic.
Plates produced with transfer type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred. Airborne dirt can create an image quality problem depending on the particular construction. In addition, because the transfer process involves melting and resolidification of material, image quality further tends to be visibly poorer than that obtainable with other methods.
3) Other patents describe lithographic printing plates comprising a support and a hydrophilic imaging layer which, upon imagewise laser exposure, becomes oleophilic in the exposed areas while remaining hydrophilic in the unexposed areas, as for example disclosed in U.S. Pat. Nos. 3,793,033; 4,034,183; 4,081,572; and 4,693,958. However, these types of lithographic printing plates suffer from the lack of a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas, with the result that image quality on printing is poor.
4) Early examples utilizing lasers used the laser to etch away material from a plate blank to form an intaglio or letterpress pattern, as for example described in U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal an oleophilic underlayer, as for example described in U.S. Pat. No. 4,054,094. These early systems generally required high-power lasers, which are expensive and slow.
More recently, other infrared laser ablation based systems for imaging hydrophilic plates have been developed. These operate by laser-mediated removal of organic hydrophilic polymers, which are coated onto an oleophilic substrate such as a polyester/metal laminate or onto an oleophilic polymer coating on a metal support. Use of these materials between the ablation coating and the heat absorbing metal support provides a thermal barrier material which reduces the amount of laser energy required to ablate or physically transform the hydrophilic surface layer, as for example described in U.S. Pat. Nos. 5,339,737; 5,351,617; 5,353,705; 5,379,698; 5,385,092; 5,440,987; 5,487,338; 5,540,150; 5,551,341; and 5,638,753; and in Canadian Pat. No. 1,050,805. Laser output either ablates one or more plate layers, or physically transforms, the oleophobic or hydrophilic surface layer, in either case resulting in an imagewise pattern of features on the plate.
One problem with this approach is that the hydrophilic non-image areas are not sufficiently durable to permit long printing runs, and are easily scratched. Also, the hydrophilic coatings are not like the traditional hydrophilic grained and anodized surfaces and generally are considered outside the mainstream of conventional printing. One other disadvantage of these plates is that they are negative working, since the portions removed by ablation are the image regions that accept ink. When lasers with a large spot size are used for imaging, the size of the smallest printed dot is as large as the spot size. Consequently, the image quality on printing is not high. For example, a 35 micron laser spot size would print its smallest dot size at 35 microns with a negative working plate. On a 200 lines per inch (lpi) halftone screen, this is equivalent to a 5% to 6% dot.
U.S. Pat. No. 5,493,971 extends the benefit of the traditional grained metal plate to ablative laser imaging and also provides the advantage of a positive working wet lithographic plate. These plates are positive working since the portions not removed by ablation are the image regions that accept ink. This construction includes a grained metal substrate, a hydrophilic protective coating which also serves as an adhesion-promoting primer, and an ablatable oleophilic surface layer. The imaging laser interacts with the ablatable surface layer, causing an ablation thereof. When lasers with a large spot size are used for imaging, the size of the smallest printed dot can be very small since the large spot size laser beam can be programmed to remove material around a very small area. Although the smallest hole in a solid printed area is large, this does not seriously affect print quality since very small holes in solids tend to fill in with ink. Consequently, the image quality on printing is high. After imaging, the plate is then cleaned with a suitable solvent, e.g., water, revealing the hydrophilic adhesion-promoting primer or the hydrophilic metal substrate. After cleaning, the plate behaves like a conventional grained metal plate on the printing press.
However, adhesion of the remaining oleophilic surface coating to the hydrophilic protective layer has proven a difficult problem to overcome. Loss of adhesion can result in a major loss of image quality. Small dots and type are often removed during development or early in the print run. Efforts to improve the adhesion of the ablatable surface coating and/or its durability to permit longer printing runs typically leads to a significant increase in the laser energy required to image the plate.
U.S. Pat. No. 5,605,780 describes a lithographic printing plate comprising an anodized aluminum support having thereon an oleophilic image-forming layer comprising an infrared-absorbing agent dispersed in a film-forming cyanoacrylate polymer binder. The hydrophilic protective layer has been eliminated. The ""780 patent describes low required laser energy, good ink receptivity, good adhesion to the support, and good wear characteristics. Print runs of more than 8,200 impressions are shown in the examples.
Despite the many efforts directed to the development of a laser imageable wet lithographic printing plate, there still remains a need for plates that require no alkaline or solvent developing solution, that look and perform like a conventional lithographic printing plate on press, that are sensitive to a broad spectrum of laser energy (700 nm to 1150 nm), that provide a high resolution image, and that will be long running on press (greater than 100,000 impressions).
One aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer characterized by the absence of ablative absorption of the laser radiation, (b) a second layer underlying the surface layer, which second layer comprises one or more polymers and is characterized by the ablative absorption of the laser radiation, and (c) a hydrophilic substrate; wherein the second layer comprises greater than 13 weight per cent of an organic sulfonic acid component based on the total weight of polymers present in the second layer. The term xe2x80x9cprinting member,xe2x80x9d as used herein, is synonymous with the term xe2x80x9cplatexe2x80x9d and pertains to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution. As used herein, for the purpose of determining the weight per cent of the organic sulfonic acid component, the term xe2x80x9cpolymersxe2x80x9d includes all the materials which are polymeric film formers, including monomeric species which polymerize or combine with a polymeric species, such as, for example, a monomeric crosslinking agent, to form the polymeric film component of the ablative-absorbing layer. In one embodiment, the organic sulfonic acid component is an aromatic sulfonic acid, preferably p-toluenesulfonic acid (PTSA). In one embodiment, the organic sulfonic acid component is a component of an amine-blocked organic sulfonic acid.
In one embodiment, the organic sulfonic acid component is present in an amount of 15 to 75 weight per cent of the total weight of polymers present in the ablative-absorbing layer of the printing member of the present invention. In another embodiment, the organic sulfonic acid component is present in an amount of 20 to 45 weight per cent of the total weight of polymers present in the ablative-absorbing layer.
In one embodiment, the thickness of the surface layer of the printing member of this invention is from about 0.1 to about 20 microns. In a preferred embodiment, the thickness of the surface layer is from about 0.1 to about 2 microns.
In one embodiment, the surface layer of the printing member of the present invention comprises a polymer and a crosslinking agent. Suitable polymers in the surface layer include, but are not limited to, polyurethanes, epoxy polymers, nitrocellulose, and polycyanoacrylates. In one embodiment, the crosslinking agent in the surface layer is a melamine. In one embodiment, the surface layer of the printing member of this invention further comprises an organic sulfonic acid component. In a preferred embodiment, the organic sulfonic acid component in the surface layer is a component of an amine-blocked p-toluenesulfonic acid.
In one embodiment, the surface layer of the printing member of this invention is further characterized by being not soluble in water or in a cleaning solution. The term xe2x80x9ccleaning solution,xe2x80x9d as used herein, pertains to a solution used to clean or remove the residual debris from the laser-ablated region of the print member of this invention and may comprise water, solvents, and combinations thereof, including buffered water solutions, as described in U.S. Pat. 5,493,971. In a preferred embodiment, the surface layer is further characterized by being not soluble in water or in a cleaning solution and by durability on a wet lithographic printing press.
In one embodiment, the ablative-absorbing second layer of the printing member of the present invention is ink-accepting. In one embodiment, the ablative-absorbing second layer is further characterized by not accepting ink and by accepting water on a wet lithographic printing press.
In one embodiment, the thickness of the ablative-absorbing second layer of the printing member of this invention is from about 0.1 to about 20 microns. In a preferred embodiment, the thickness of the ablative-absorbing second layer is from about 0.1 to about 2 microns.
In one embodiment, the ablative-absorbing second layer of the printing member of this invention comprises an infrared sensitizer. In one embodiment, the infrared sensitizer in the ablative-absorbing second layer is a carbon black. In a preferred embodiment, the carbon black of the infrared sensitizer of the ablative-absorbing layer comprises a sulfonate group on the surface of the carbon black, and most preferably the carbon black is CAB-O-JET 200. In one embodiment, one or more polymers of the ablative-absorbing second layer of the printing member of the present invention further is a crosslinking agent. Suitable polymers in the ablative-absorbing second layer include, but are not limited to, nitrocellulose; polycyanoacrylates; polyurethanes; polyvinyl alcohols; polyvinyl acetates; polyvinyl chlorides; and copolymers and terpolymers thereof. In one embodiment, one or more of the polymers of the ablative-absorbing second layer is a hydrophilic polymer. In one embodiment, the crosslinking agent of the ablative-absorbing second layer is a melamine.
In one embodiment, the ablative-absorbing second layer of the printing member of this invention is characterized by being not soluble in water or in a cleaning solution.
Suitable substrates for this aspect of the printing member of the present invention are hydrophilic and include, but are not limited to, metals, papers, and polymeric films.
Suitable polymeric films for the substrate include, but are not limited to, polyesters, polycarbonates, and polystyrene. In one embodiment, the polymeric film of the substrate is treated to make it hydrophilic. In one embodiment, the substrate is a polyester film, preferably a polyethylene terephthalate film. Suitable metals for the substrate include, but are not limited to, aluminum, copper, chromium, and steel. In a preferred embodiment, the metal of the substrate is grained, anodized, silicated, or a combination thereof. In a preferred embodiment, the substrate is aluminum.
Another aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer characterized by the absence of ablative absorption of the laser radiation, as described herein; (b) a second layer under the surface layer, which second layer comprises one or more polymers and is characterized by the ablative absorption of the laser radiation, as described herein; (c) a hydrophilic third layer underlying the second layer, which third layer is characterized by the absence of ablative absorption of the laser radiation; and (d) a substrate; wherein the second layer comprises greater than 13 weight per cent of an organic sulfonic acid component, as described herein, based in the total weight of polymers present in the second layer. In one embodiment, the thickness of the third layer of the printing member of this invention is from about 1 to about 40 microns. In one embodiment, the thickness of the third layer is from about 2 to about 25 microns.
In one embodiment, the hydrophilic third layer of the printing member of the present invention comprises a hydrophilic polymer and a crosslinking agent. Suitable hydrophilic resins for the third layer include, but are not limited to, polyvinyl alcohols and cellulosics. In a preferred embodiment, the hydrophilic polymer of the third layer is polyvinyl alcohol. In one embodiment, the crosslinking agent is a zirconium compound such as, for example, ammonium zirconyl carbonate.
In one embodiment, the hydrophilic third layer of the printing member of this invention is characterized by being not soluble in water or in a cleaning solution. In one embodiment, the hydrophilic third layer is characterized by being not excessively soluble in water or in a cleaning solution.
Suitable substrates for this aspect of the printing member of the present invention, which printing member comprises a hydrophilic polymeric or third layer interposed between the ablative-absorbing layer and the substrate, are either hydrophilic or non-hydrophilic/ink-accepting and include, but are not limited to, metals, papers, and polymeric films. Suitable polymeric films for the substrate include, but are not limited to, polyesters, polycarbonates, and polystyrene. In one embodiment, the polymeric film of the substrate is treated to make it hydrophilic. In one embodiment, the substrate is a polyester film, preferably a polyethylene terephthalate film. Suitable metals for the substrate include, but are not limited to, aluminum, copper, chromium, and steel. In a preferred embodiment, the metal of the substrate is grained, anodized, silicated, or a combination thereof. In a preferred embodiment, the substrate is aluminum.
One aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer characterized by the absence of ablative absorption of the laser radiation, as described herein; (b) a second layer underlying the surface layer, which second layer comprises one or more polymers and is characterized by the ablative absorption of the laser radiation, as described herein; and (c) a hydrophilic substrate, as described herein; wherein interposed between the second layer and the hydrophilic substrate is a primer layer comprising an adhesion-promoting agent. The primer layer is characterized by the absence of ablative absorption of the laser radiation. In one embodiment, the primer layer is characterized by being hydrophilic, by the absence of ablative absorption of the laser radiation, by being not ablated by the ablative absorption of the second or ablative-absorbing layer, and by being not soluble in water. In one embodiment, the primer layer is further characterized by being not removed by the ablative absorption of the second layer followed by a cleaning step with water or a cleaning solution to remove any residue of the ablative absorption of the second layer from the surface of the primer layer.
In one embodiment, the adhesion-promoting agent of the primer layer comprises a zirconium compound. In one embodiment, the adhesion-promoting agent of the primer layer comprises ammonium zirconyl carbonate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium propionate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium oxide. In one embodiment, the primer layer is an inorganic gel layer, preferably an inorganic gel layer comprising a zirconium oxide gel.
In another embodiment, the adhesion-promoting agent of the primer layer comprises an organic sulfonic acid component, preferably an aromatic sulfonic acid, and more preferably p-toluenesulfonic acid. In one embodiment, the organic sulfonic acid component in the primer layer interposed between the ablative-absorbing second layer and the hydrophilic substrate is present in an amount of 2 to 100 weight per cent of the primer layer, preferably in an amount of 50 to 100 weight per cent of the primer layer, and most preferably in an amount of 80 to 100 weight per cent of the primer layer.
In one embodiment, the thickness of the primer layer interposed between the second layer and the substrate is from about 0.01 to about 2 microns, and preferably from about 0.01 to about 0.1 microns.
Another aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer characterized by the absence of ablative absorption of the laser radiation, as described herein; (b) a second layer underlying the surface layer, which second layer comprises one or more polymers and is characterized by the ablative absorption of the laser radiation, as described herein; (c) a hydrophilic third layer underlying the second layer, which third layer is characterized by the absence of ablative absorption of the laser radiation, as described herein; and (d) a substrate, as described herein; wherein interposed between the second and the third layer is a primer layer comprising an adhesion-promoting agent. The primer layer is characterized by the absence of ablative absorption of the laser radiation. In one embodiment, the primer layer is characterized by being hydrophilic, by the absence of ablative absorbtion of the laser radiation, by being not ablated by the ablative absorption of the second or ablative-absorbing layer, and by being not soluble in water. In one embodiment, the primer layer is further characterized by being not removed by the ablative absorption of the second layer followed by a cleaning solution to remove any residue of the ablative absorption of the second layer from the surface of the primer layer.
In one embodiment, the adhesion-promoting agent of the primer layer comprises a zirconium compound. In one embodiment, the adhesion-promoting agent of the primer layer comprises ammonium zirconyl carbonate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium propionate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium oxide. In one embodiment, the primer layer is an inorganic gel layer, preferably an inorganic gel layer comprising a zirconium oxide gel. In another embodiment, the adhesion-promoting agent of the primer layer comprises an organic sulfonic acid component, preferably an aromatic sulfonic acid. In one embodiment, the organic sulfonic acid component in the primer layer interposed between the second and the third layer is present in an amount of 2 to 100 weight per cent of the primer layer, preferably in an amount of 50 to 100 weight per cent of the primer layer, and most preferably in an amount of 80 to 100 weight per cent of the primer layer.
In one embodiment, the thickness of the primer layer interposed between the second and the third layer is from about 0.01 to about 2 microns, and preferably from about 0.01 to about 0.1 microns.
Another aspect of the present invention pertains to methods of preparing a positive working, wet lithographic printing member imageable by laser radiation, as described herein.
In a preferred embodiment, the method of preparing a positive working, wet lithographic printing member imageable by laser radiation comprises (a) providing a grained and anodized metal substrate, (b) coating a hydrophilic polymer layer on the substrate, which polymer layer comprises a hydrophilic polymer and a crosslinking agent and subsequently curing the polymer layer, (c) coating an intermediate layer over the polymer layer, which intermediate layer comprises an ablative-absorbing sensitizer, a hydrophilic polymer, and a crosslinking agent, and subsequently curing the intermediate layer to form an ablative-absorbing layer, and (d) coating an ink-accepting surface layer over the intermediate layer, which surface layer comprises a polymer and a crosslinking agent, and subsequently curing to form a thin durable ink-accepting surface layer; wherein the intermediate layer further comprises greater than 13 weight per cent of an organic sulfonic acid component based on the total weight of polymers present in the second layer. In a more preferred embodiment, the surface layer of the printing member further comprises an organic sulfonic acid component.
Yet another aspect of this invention pertains to methods of preparing an imaged wet lithographic printing plate comprising (a) providing a positive working, wet lithographic printing member, as described herein; (b) exposing the printing member to a desired imagewise exposure of laser radiation to ablate part of the ink-accepting surface layer of the printing member and to ablate a part of the ablative-absorbing second layer of the printing member to form a residual composite layer on the hydrophilic substrate or, alternatively, on the hydrophilic third layer if one is present; and (c) cleaning the residual composite layer from the hydrophilic substrate or, alternatively, from the hydrophilic third layer if one is present underlying the ablative-absorbing second layer of the printing member, which cleaning is done with water or with a cleaning solution; wherein the ink-accepting surface layer of the printing member is not soluble in water or in the cleaning solution.
The lithographic printing members of this invention are positive working plates. The second layer, which is ablative absorptive, and the surface layer, which is ink-accepting, oleophilic, hydrophobic, and durable, are ablated and substantially completely removed in a post-imaging cleaning step in the regions exposed to the laser radiation so that the non-exposed regions serve as the ink-transferring surface in lithographic printing. After imaging, in a preferred embodiment, when a hydrophilic third layer is present underlying the ablative-absorbing second layer, a crosslinked hydrophilic polymeric third layer remains on the plate in the laser imaged areas, along with a quantity of ablation by-products or residual composite layer, typically loosely bound to the hydrophilic third layer. The hydrophilic third layer enhances the clean-up of the by-product or residual composite layer, since it is much easier to remove from the hydrophilic third layer than from a hydrophilic substrate, such as a grained and anodized aluminum surface. One advantage of the present invention is that the lithographic printing member or plate can be used to print immediately, since fountain solution will easily clean the ablation debris or residual composite layer from the plate. In the course of a long printing run, the hydrophilic third layer, when present, typically is not solubilized, and non-hydrophilic substrates may be utilized. Optionally, the hydrophilic third layer may only very slowly solubilize, and hydrophilic substrates are then preferred so that, if the hydrophilic third layer is removed by solubilization, the hydrophilic substrate is uncovered underneath. In this latter case, the printing characteristics of the non-image areas are not affected since one hydrophilic layer is merely exchanged for another. On the other hand, the hydrophilic third layer under the non-exposed image areas of the present invention provides an excellent adhesion primer for this image layer since it is nearly impossible to undercut through solubilization, particularly when the hydrophilic third layer is crosslinked.
The superiority of the lithographic printing member of the present invention over those previously known is particularly manifest in its ability to be imaged rapidly with relatively inexpensive diode lasers with large spot sizes, its ease of cleaning, its excellent image resolution and printing quality, its resistance to water, alkali, and solvents which provides excellent durability and image adhesion on the printing press, and its low cost of manufacture.
The presence of greater than 13 weight per cent of an organic sulfonic acid component based on the total polymers present in the ablative-absorbing second layer and, optionally, the presence of an organic sulfonic acid component in the ink-accepting surface layer, in the hydrophilic third layer when present, and in a primer layer when present, significantly enhances the combination of high laser sensitivity, high image resolution, ease of cleaning the residual composite layer formed in the laser-exposed areas, and the excellent durability, adhesion, and water and fountain solution resistance of the ink-accepting image areas on the printing press that are desired in lithographic printing utilizing direct imaging by lasers.
Yet another aspect of the present invention pertains to a positive working, wet lithographic printing member comprising an ablative-absorbing layer as an ink-accepting surface layer, wherein the ablative-absorbing layer comprises greater than 13 weight per cent of an organic sulfonic acid component, as described herein, based on the total weight of polymers present in the ablative-absorbing layer. The high weight per cent of an organic sulfonic acid component in the ablative-absorbing surface layer provides the lithographic printing member with the combined benefits of rapid imaging, ease of cleaning the residual non-ablated debris in the laser imaged areas, excellent image resolution and quality, and resistance to water for excellent durability and image adhesion on the printing press, but without requiring the additional non-ablative absorbing, ink-accepting overcoat surface layer of other aspects of this invention. Thus, another aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer, which surface layer comprises one or more polymers and is characterized by the ablative absorption of laser radiation, as described herein; (b) optionally, a hydrophilic polymeric layer, which hydrophilic polymeric layer underlies the surface layer and is characterized by the absence of ablative absorption of the laser radiation, as described herein; and, (c) a substrate, as described herein; wherein the surface layer further comprises greater than 13 weight per cent of an organic sulfonic acid component based on the total weight of polymers present in the surface layer.
Further, still another aspect of the present invention pertains to a positive working, wet lithographic printing member imageable by laser radiation comprising (a) an ink-accepting surface layer, which surface layer comprises one or more polymers and is characterized by the ablative absorption of the laser radiation, as described herein; (b) optionally, a hydrophilic polymeric layer, which hydrophilic polymeric layer underlies the surface layer and is characterized by the absence of ablative absorption of the laser radiation, as described herein; and, (c) a substrate, as described herein; wherein interposed between the hydrophilic polymeric layer and the surface layer is a primer layer comprising an adhesion-promoting agent. The primer layer is characterized by the absence of ablative absorption of the laser radiation. In one embodiment, the primer layer is characterized by being hydrophilic, by the absence of ablative absorption of the laser radiation, by being not ablated by the ablative absorption of the surface or ablative-absorbing layer, and by being not soluble in water. In one embodiment, the primer layer is further characterized by being not removed by the ablative absorption of the surface layer followed by a cleaning step with water or a cleaning solution to remove any residue of the ablative absorption of the surface layer from the surface of the primer layer. In one embodiment, the adhesion-promoting agent of the primer layer comprises a zirconium compound. In one embodiment, the adhesion-promoting agent of the primer layer comprises ammonium zirconyl carbonate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium propionate. In one embodiment, the adhesion-promoting agent of the primer layer comprises zirconium oxide. In one embodiment, the primer layer is an inorganic gel layer, preferably an inorganic gel layer comprising a zirconium oxide gel. In another embodiment, the adhesion-promoting agent of the primer layer comprises an organic sulfonic acid component, preferably an aromatic sulfonic acid. In one embodiment, the organic sulfonic acid component in the primer layer interposed between the hydrophilic polymeric layer and the ablative-absorbing surface layer is present in the amount of 2 to 100 weight per cent of the primer layer, preferably in an amount of 50 to 100 weight per cent of the primer layer, and most preferably in an amount of 80 to 100 weight per cent of the primer layer. In one embodiment, in addition to the presence of the primer layer, the ablative-absorbing surface layer further comprises greater than 13 weight per cent of an organic sulfonic acid component based on the total weight of polymers present in the ablative-absorbing surface layer.
Yet another aspect of this invention pertains to methods of preparing an imaged wet lithographic printing member comprising (a) providing a positive working, wet lithographic printing member comprising an ink-accepting surface layer, which surface layer comprises one or more polymers and is characterized by the ablative absorption of laser radiation, as described herein, (b) exposing the printing member to a desired imagewise exposure of laser radiation to ablate part of the ablative-absorbing surface layer of the printing member to form a residual composite layer on the hydrophilic substrate or, alternatively, on the hydrophilic polymeric second layer if one is present, and (c) cleaning the residual composite layer from the hydrophilic substrate or, alternatively, from the hydrophilic polymeric second layer if one is present underlying the surface layer of the printing member, which cleaning is done with water or with a cleaning solution; wherein the ablative-absorbing, ink-accepting surface layer of the printing member is not soluble in water or in the cleaning solution.
As one of skill in the art will appreciate, features of one embodiment and aspect of the invention are applicable to other embodiments and aspects of the invention.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.